1. Field of the Invention
The present invention relates to a table feed apparatus which is applied to a machine tool or the like.
2. Description of the Prior Art
Table feed apparatus have heretofore been known in which a movable table is slideably carried by a fixed bed. In general, the feed mechanism for driving the movable table is constituted by a combination of a ball screw and nut assembly and rotary motors such as servo motors, stepping motors or the like, and the rectilinear motion of the movable table is normally controlled by utilizing the rotary motion of the ball screw and nut assembly, thereby effecting high precision positioning of the movable table. However, such a conventional type of apparatus suffers from a problem in that, due to the backlash of the ball screw and nut assembly, the starting and stopping response characteristics are undesirably impaired in such a way that it is difficult to precisely control the position of the movable table. If the ball screw and nut assembly is arranged to have a small pitch in order to attain high precision positioning, the speed of feed of the apparatus unavoidably slows down. It is also to be pointed out that the size of the apparatus as a whole is increased due to the need to mount thereon the ball screw and nut assembly and rotary motors.
In order to overcome these problems, various proposals have been made with respect to table feed apparatus employing a linear motor. The arrangement which incorporates a linear motor is effective in that the response of movement of the movable table is superior and the table can be fed at high speed due to the elimination of a driving-force transmission mechanism employing mechanical means such as a ball screw and nut assembly. Furthermore, the arrangement is effective in that the size of the apparatus can be decreased since the linear motor has a movable member and a fixed member interposed between the movable table and the fixed bed.
However, the above-described conventional type of table feed apparatus which includes a linear motor is normally arranged in such a manner that the movable table travels on the fixed member secured to the fixed bed, so that the range in which the movable table can travel is limited within the range corresponding to the entire length of the fixed member. For this reason, in order to enable the movable table to travel a long distance, it is necessary to prepare a long fixed member. On the other hand, since teeth on the fixed member are formed through the etching of a steel sheet or the like, if the fixed member is to be increased in length, the etching bath employed must be lengthened accordingly, thus leading to an increase in the scale of the production facility and in production costs. Thus, the prior art involves difficulties in that the length of the etching bath is generally limited, and, as the fixed member per se becomes longer, the production cost is increased. In order to solve these problems, one method has been considered in which a plurality of short fixed members are jointed so as to form a long fixed member. However, in this method, it is difficult to unite the joints of the short fixed members with high precision exactly the same as the pitch of the fixed teeth. For example, even if the movable member is arranged to travel at 1 mm pitch, when the member passes over each joint it tends to run over the joint at speeds substantially corresponding to 1.5 mm pitch or less than 1 mm pitch since each pitch as defined by the adjacent joints is prone to be varied producing irregular lengths such as 1.5 mm or less than 1 mm rather than the normal dimension of 1 mm. This disadvantage raises certain problems such as the deterioration of the precision with which the movable table travels.